The present invention relates to sintering preforms and to a method for connecting at least two components in which these sintering preforms are used.
In the field of power electronics, the connecting of at least two components that have high sensitivity to pressure and temperature, for example substrates having electronic components such as LEDs or very thin silicon chips, presents a particular challenge.
For this reason, substrates having such pressure- and temperature-sensitive components are often connected by gluing. Suitable conductive glues customarily contain silver particles, thermosetting polymers, and reactive thinners. However, the gluing technology has the disadvantage that it creates contact points between the substrate and the component that have insufficient heat conductivity or electrical conductivity.
In order to solve this problem, it has been proposed to connect components to substrates by sintering.
These sintering methods customarily use pastes consisting of the metal powder to be sintered and a solvent.
For this purpose, German Patent DE 34 14 065 C2 proposes (i) the application of such a paste onto the surface of an electronic component or substrate to be connected, (ii) application of the component onto the substrate, wherein the paste is situated between the component and the substrate, (iii) expelling the solvent from the composite created in this way, and (iv) sintering the composite.
This sintering method achieves the reliable connection of the electronic component to the substrate. However, it has turned out to be disadvantageous that the solvent is not expelled until after the composite of the electronic component and the substrate has already been created. Because in this composite the paste containing the solvent already stands in contact with the surfaces of the electronic component and of the substrate to be connected, a simple and rapid degassing is no longer possible, so that the composite has to be dried for a longer period of time.
In order to solve this problem, according to European Patent EP 0 242 626 B1 the paste is applied onto the surface of the electronic component or of the substrate to be connected and is dried immediately thereafter. Only after the drying is the component placed onto the substrate, wherein the dried paste is situated between the electronic component and the substrate. Thereafter, the composite created in this way is sintered.
A further development of this method is known from German Patent DE 10 2004 019 567 B3. That document proposes that a paste be applied as a layer onto a carrier foil and dried. Thereafter, one or more components are placed onto the layer of dried paste. Pressure is thereafter applied to the composite made up of the component, the layer of dried paste, and the carrier foil, in order to increase the adhesive force between the dried paste and the component so that the dried paste adheres to the component and can thus be lifted off from the carrier foil. The component with the dried paste can thereafter be positioned on the substrate, and the composite made up of the substrate, component, and the layer of dried paste situated between them can be sintered.
From an economic point of view, this method has turned out to be advantageous, because it enables a rational and at least partly parallel processing of a plurality of components, as well as a structured design of the sintering layer.
However, here it is disadvantageous that this sintering method, like other conventional sintering methods, requires either a high process pressure (for example greater than 30 MPa) or else a high process temperature (greater than 250° C.). These conditions often cause damage to the components to be connected, so that conventional sintering methods must be ruled out for many applications.
German published patent application DE 10 2007 046 901 A1 proposes a sintering technique that succeeds in creating connecting layers for use in power electronics, having very good electrical and thermal conductivity. In this sintering method, a metal paste is used that contains, in addition to an alcoholic solvent, a silver compound that decomposes to form elemental silver at 300° C. These metal pastes enable a reduction of the process pressure to less than 3 bars and a reduction of the process temperature to less than 250° C. This sintering technique represents a large leap in quality in the connection of substrates having pressure- and temperature-sensitive components.
However, for many applications it would be desirable to further lower the process temperature. This would place a lower load on the components to be connected, and would thus provide a further increase in quality of components in the area of power electronics. In addition, a further lowering of the process temperature would create significant savings in energy costs.